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Thursday, July 19, 2007

Big severe storm day today in AZ?

Prepared at 8:00 am MST, 19 July 2007

The summer thunderstorm season in southern Arizona has been very slow and sparse to date - except in the far south borderlands. The last few days have been characterized by little CAPE at low elevations, with storms mostly clinging to the mountains in the Tucson area. The low-level moisture levels have been adequate to support some storms, and it has generally come from weak pushes of higher dewpoint air into southern AZ, some nice outflows from storm complexes to the east and south. these have combined with abundant recyling to keep the IPW values in the 30 to 40 mm range. Yesterday, with stronger steering-level winds finally, a storm cell moved off the western end of the Catalina Mountains and produced a wet downburst in Oro Valley. Lots of thunder and wind and dust here, and then a brief burst of rain that left 0.05" in the gauge. We still await a serious push of deep sub-tropical moisture into the state.

But, lots of interesting things are finally happening - a large, Sonoran MCS decayed over the central GoC this am and it appears to have started what will finally be a serious push of very moist subtropical air into southern AZ Doppler radar VAD winds at Yuma show nice SSE winds up to 4,000 ft. The upper-level inverted trough over northern Mexico appears will affect southern AZ late pm - early night today with nice difluence aloft. By tomorrow evening it will be right overhead, with the difluence weakened and shifted to NW AZ. During this period there is upper-level ridging over a subtropical circulation that is located south of the mouth of GoC.

This system is predicted to strengthen considerably by the NAM and may becomea TS? Regardless, it is perfectly positioned to intensify the surge of moisture into AZ for the next 60 hours!

The NAM suggests a not very favorable upper-level shear profile this afternoon with easterlies of nearly equal strength from 500 to 200 mb, but everything else looks like it's coming together, and the last two days the model predicted winds have been erratic over this corner of state.

So which day will be the big day? I'm going to go with:

Likely big severe tstm day today, and then two days of good, heavy rain storms.